Indicating fuse



W. C. LINTON INDICATING FUSE July 2, 1940.

Filed March 6, 1939 2 Sheets-Sheet l July 2, 1940- w. c. LINTON 2,296,781

INDICATING FUSE Filed March 6, 1959 2 Sheets-Sheet 2 Patented July 2, 1940 UITED 4STATES PATENT OFFICE Application March 6, 1939, Serial No. 260,155

5 Claims.

The present invention relates to fuses for electrical circuits capable, upon the rupture of the fuse link or elements thereof, of giving or emitting a visible signal or indication and more particularly to plug type fuses wherein the indicator may be housed entirely within the plugior casing of the fuse, having among its principal objects to produce an inexpensive indicating plug type iuse which will be commercially desirable.

Another equally important object of the present invention is to provide a leak resistance for electrical circuits having glow lamp indicators therein which will not only provide means for resisting the ilow of current through the circuit and permit suillcient leakage of current to render the lamp operative, but will provide means for electrically connecting the lead wires of the lamp and electrical circuit without the use of solder or other connecting elements or substances thereby materially reducing the cost associated with assembling and installing indicating glow lamps within an electrical circuit.

A still further object of the present invention is to provide an electrical element having certain characteristics of adapting it for use in providing resistance in electrical systems which can be easily and readily handled, molded into various shapes and forms, without pressure or without heat and pressure, have a great aillnity towards all kinds of metals and one which when molded or shaped into its tlnal form bonds solidly, will not break down with age, repels water and moisture and will resist heat to a degree beyond its operating temperatures to which it is suited, that is, a leak resistance for indicating electrical fuses.

Other objects of the invention will be in part obvious and in part pointed out hereinafter.

In order that the invention and its mode of operation may be readily understood by persons skilled in the art, I have in the accompanying drawings and in the detailed following description based thereupon, set out an embodiment of the same.

In the drawings:

Fig. 1 is a plan view of the fuse embodying the present invention.

Fig. 2 is a vertical transverse section through the same on the line 2--2 of Fig. 1 in the dlrection of the arrow points.

Fig. 3 is a vertical transverse section taken at right angles to Fig. 2.

Fig. 4 is a plan view of the fuse casing.

Fig. 5 is a vertical transverse section through the same taken on the line 5--5 of Fig. 4.

Fig. 6 is a vertical transverse section oi the (Cl. 20G-121) iuse casing taken on the line 6 6 of Fig. 4 in the direction of the arrow points 8.

Fig. 7 is a similar vertical transverse section taken on the line 6-8 of Fig. 4 but in the direction of the arrow points 1. 5

Fig. 8 is a plan view of the paper partition or divider.

Fig. 9 is a vertical sectional view through a fuse plug showing a slightly modied form of the invention.

-Fig. 10 is a similar view taken at right angles to that shown in Fig. 9, and

Fig. 11 is another slightly modiled form of the invention.

Referring now more particularly to the drawl5 ings wherein like and corresponding parts are designated by similar reference characters throughout the several views, there is herein shown a cup-shaped casing I molded from porcelain or other insulating material having an ex- I0 plosive chamber 2 formed therein with its upper end open and its lower end communicating with an opening 3. A vent opening 4 is formed within the casing having communication with this explosive chamber 2 and the outer side or ll tace of the casing. Molded with the outer surface of the casing are screw threads 5 and an annular flange 6. The lower edge of this annular flange I has the equally spaced recesses 'l formed therein.

Seated within the opening 3 is a rivet 8 forming the central contact terminal of the fuse and engaging the threads 5 is a threaded contact shell 9. A fuse link I0 has one end soldered to the rivet 8 and its opposite end extending 85 through the vent opening 4 where its extremity is retained in electrical contact with the shell S. The upper open end of the casing is closed by means of a mica disc Ii, which latter is retained in place upon the fuse casing by means 40 of a metal shell I2. The outer edge of this shell i2 is crimped about the ange 6 of the casing for retaining the same thereupon and certain portions of the shell i2 are seated within the recesses 1 to prevent the rotation of the shell 45 upon .the casing.

'I'he foregoing describes the parts of a conventional plug type fuse for lighting systems or other electrical circuits so that should a fusible link i0 become blown or ruptured, caused by 50 a short circuit within the lighting system, one can readily determine the condition of the fusible link lli by looking through a window formed by the mica disc il.

By close inspection, one can tell the condi- 5d tion of the fusible link by looking through the mica window Il providing suicient light is available and it is the aim of the present invention to provide a practical lamp indicator for this type of plug fuses which will prainly indicate and readily make visible the blown or ruptured condition of the fuse link I0, indicating that the fuse must be repaired or replaced to render the lighting circuit again useful.

Ordinary incandescent or neon lamps will not operate as a practical indicator for fuses nor will other indicating means, all of which require voltage for their successful operation, as a fuse is for the purpose of cutting out an electrical circuit and should a lamp or other indicating means, which require voltage, be employed, they would defeat the very purpose of the fuse by permitting the current to pass to the point where the short circuit occurs. Also, this lamp or other indicator must be very small in size as to be placed within a shell or casing of the fuse plug but I have found that a miniature glow lamp containing a mixture of inert gases which pass electric currents between the electrodes having a relatively low potential difference, the electrodes of which are in series with a leak resistance, both the lamp and the leak resistance being in series with the conducting terminals of this fuse, but mounted in parallel so far as the fusible element is concerned, is well suited as a practical lamp indicator for fuses.

The low potential mixture of inert gases employed in the manufacture of these lamps preferably comprises neon combined with a relatively small amount of argon. Any suitable gas or other substance is added for the purpose of purifying the inert gases whereby the globe of the lamp may be made in small sizes and the lamp may have a relatively long life. Such a lamp is herein shown and indicated by the numeral I3 and the electrodes of which have lead wires I4 and I5 extending therefrom. The outer extremity of the wire I5 is electrically connected to the rivet terminal 8 of the fuse Whereas, the outer end of the wire I4 is embedded within a leak resistance I6.

Leak resistances have been commonly used in connection with this type of glow lamp for permitting sufficient current to pass to the electrodes of the lamp for causing the latter to glow and after once being lighted, to permit sufficient leakage as to cause the lamp to remain lighted as long as a small amount of current is available and during the usual life of the lamp. Such leak resistances also prevent excessive currents from passing around or through the fuse after the fuse link has become ruptured as would cause a fire at the point where the short circuit occurs in the lighting circuit but these leak resistances do not serve as a means for regulating a suitable increase or decrease in'voltage through the lighting circuit as to regulate the amount of voltage as is required for ordinary types of neon or incandescent lamps. This glow lamp will strike and stay lighted when applied to high potential circuits or low potential circuits of not lless than forty volts. This lamp, when used in connection with high or low potential circuits, would soon blow out or become inoperative if some means such as a leak .resistance were not employed which would only allow an infinitesimal amount of current to pass to the lamp for causing the latter to remain lighted and at the same time, such leak resistance will prevent any noticeable current from passing through the electrical circuit to which the lamp is applied. These leak resistances must be as high as fifty thousand ohms when used on the average lighting circuit of volts and their ohms resistance increased according to the potential of the circuit to which it is applied. This lamp may be applied to high potentialcircuits as high as six hundred sixty volts but the resistance must be approximately fifteen million ohms. Such high leak lresistance will permit suiilcient current to pass to the lamp for causingy the latter to glow but at the same time, whether applied to a high or low potential circuit, will prevent a sufficient amount of current from passing through the circuit as would cause damage at the point of the short circuit.

The relatively small amount of current required to render this lamp luminous enables mev to use such a very high resistance, so that in the normal operation of the lamp, no noticeable current can pass through this resistor I6 as would render the fuse unsuited for the purpose for which it is intended. Furthermore, the electric power required to operate the lamp herein illustrated and described is approximately .02 of a Watt, and when operated, gives the characteristie neon red light. Ordinary incandescent 0r neon lamps arranged in parallel in an electric circuit will not permit of more than fifty per cent variation in the voltage, whereas, the lamp herein illustrated and described, operates on any voltage above a certain minimum amount. With an ordinary lamp used for a similar purpose, it is well known that the resistance must be of a suitable current carrying capacity, whereas, with the glow lamp herein employed, the current consumption is practically nil.

The leak resistance I6 herein employed is prepared from. a dielectric composition composed principally of a thermo-plastic condensation derivative of rubber prepared by the treatment of rubber with a halide of an amploteric metal or chlorostannic acid and a conductive substance such as finely divided metal, powdered graphite or other form of powdered carbon. I preferably employ a powdered Pliolite as a binder, powdered graphite or flocculated carbon black as the conductive material and a finely divided insulating material such as sand, clay, mica or silica. These powdered materials are then thoroughly mixed together. terials is about the same as graphite or carbon black and give a greater conductivity than coarser forms of carbon and will readily mix with this dry, powdered, or milled rubber derivative.

As far as I can learn, Pliolite is a thermoplastic condensation derivative of rubber prepared by the treatment .of rubber with a halide of an amphoteric metal or chlorostannic acid sold under this trade name by the Goodyear 'Iire and Rubber Company of Akron, Ohio, prepared according to one of U. S. Patents Nos. 1,377,152, 1,797,188, 1,846,247 and 1,853,334 and exemplified by Thies and Clifford in Industrial and Engineering Chemistry, vol. 28, pages 123-129 (February, 1934) and H. A. Brunson, L. B. Sebrell and W. C. Calvert, Industrial and Engineering Chemistry, September, 1927, pages 1033 to 1037, entitled New chemical reactions of rubber hydrocarbons with metallic halides. These rubber derivatives are known by the trade names Pliolm, "Plioform and Pliolite prepared in sheet, crushed, liquid, and powdered form, but only the powdered or milled form is suitable for my leak resistance.

Therefore, the binder for the resistor I6 is prepared for the trade ln a dry, powdered form and The fineness of these powdered masold under the trade name "Pliolite and will hereina-fter be referred to as Pliolite, rubber derivative or thermo-plastic rubber derivative.

The carbon black employed by me is that prepared for the trade by the United Carbon Company of New York, N. Y., in a dry, powdered, or pellet form and sold under the trade name Kosmob1le."

The graphite employed by me is that prepared for the trade by the Acheson Graphite Corp. of Cleveland, Ohio, and known as Acheson Graphite Grade No. 2301. It is made from a petroleum coke and is said to pass through a screen having 200 meshes per square inch and will hereinafter be referred to as powdered graphite.

In the fabrication of electrical resistors, binders, such as phenol-condensation products, thermo-plastic substance made from rubber and vulcanizable rubber have heretobefore been employed, but only this particular form of powdered Pliolite is suitable as a binder in the manufacture of my resistors, as it has, in addition to the well known chemical reactions, physical and electrical characteristics of similar thermo-plastic rubber derivatives, a great ailinity to all kinds of metal, readily mixing with granular, conductive and insulating substances and may be formed into a homogeneous mass without pressure and/or without heat and pressure.

By heating this mixture of Pliolite, sand and graphite or carbon above 300 F. a melt is formed wherein the Pliolite will hold the conductive material in suspension therein and as the Pliolite will adhere tenaciously to any metal, it will readily stick to and form a bond with the lead wire I4 and the brass contact shell 9. This Pliolite will a-lso hold the carbon in electrical contact with the lead wire I4 and the shell 9, thereby eliminating the necessity of soldering these terminals to the leak resistance. When this heated mixture of Pliolite and carbon is allowed to cool, a hard, solid mass results which will not crumble or break during the handling of the fuse and, further, it will securely fasten the wire I4 and shell 9 thereto.

'Ihis resinous rubber derivative may be mixed with a plasticizer, such as diamyl naphthalene or butyl stearate to render the same plastic" and it Y is also soluble in toluol, benzene and other hydrocarbons, but it is insoluble in alcohols, acetones and ethers. Therefore, instead of heating this mixture to form a plastic mass, I can and do add a suitable solvent thereto to form a melt or soft mass which can be readily molded into any shape desired. While in this plastic condition, the lead wire I4 can be embedded therein and the contact terminal 9 attached thereto and upon the complete evaporation of the solvent, a hard mass results which will not break down with age and which will further bond solidly to the metal conductors attached thereto or embedded therein.

A specic example of this dielectric compound may consist of eighty parts by weight of the powdered rubber derivative and twenty parts by weight of powdered graphite thoroughly mixed together and then heated to about 350 F. to form a plastic mass. The conductivity of the mass may be accordingly determined, over a wide range, by the proportion of carbon black, graphite, metai or other conductive substances employed and the different conductive properties also may be had according to the neness of the particles of conductive material used and the thoroughness of the mixing. By thoroughly mixing this dielectric thermo-plastic resinous powder with the comminuted conductive mate-H rial, the particles of this latter a-re caused to distribute themselves quite uniformly throughout the mass, with at least a part of them in conductive relation to eachother, and, the number of particles being very great, either the law of averages so far applies as to produce substantially the same degree of conductivity in successive masses prepared in the same way and equally mixed, or the properties of the plastic and particles result in such substantially uniform distribution and arrangement of the latter, as to provide dependable uniformity of conductivity throughout a given mass or in respective masses similarly prepared.

Instead of applying heat to this mixture of eighty parts of the powdered Pliolite and twenty parts of powdered graphite, it is equally desirable to add thereto diamyl naphthalene, toluol, benzene or other coal tar derivative or petroleum solvents wherein a plastic composition is produced which may be formed or molded into any shape required and will remain in the shape so formed after the solvent has evaporated.

In order to apply within a 4plug type fuse as herein described such a miniature glow lamp and leak resistance, I form within the casing I the recesses I1 and I8 which are a-rranged diametrically opposite one another and at right angles to the vent opening 4. The pointed end of the lamp is then seated within the recess I1 and the flat end thereof will seat itself within the recess I8. whereby the entire globe of the lamp is housed within the fuse casing below the mica disc or window II. The lead wire I5 of the lamp is seated within a groove I9 formed within the casing I a-nd which extends from the recess I8 to the rivet receiving opening 3, whereas, the other lead fire I4 of the lamp projects within a vertically extending opening 20 formed within the fuse casing. This latter opening 20 extends from the bottom of the recess I8 and 'opens into a recess 2l formed within the outer wall of the fuse casing. The resistor composition I6 is inserted within the opening 20 and the recess 2| for providing an electrical connection between the lead wire I4 and the outer contact terminal 9.

From this arrangement, it is quite obvious that the fuse link III- is in series withthe terminals 8 and 9 and also the lamp I3 and leak resistance are in series with one another and the terminals 8 and 9 but in parallel to the fuse link I0, whereupon, should the fuse link become blown or ruptured, the current will pass to the lamp, causing it first to strike and as only a small quantity of current will be permitted to pass through the resistor, the lamp will remain lighted for indicating through the mica window II the destroyed condition of the fuse link I0.

In Fig. 8 of the drawings, I have shown in plan view a paper disc 22 which may have an outer refiecting surface and has a portion thereof cut away, as shown. This paper disc may be pleated along the dotted lines indicated in Fig. 8, whereby it may be seated within the explosive chamber 2 of the fuse casing in order to separate the fusible link I from the lamp and lead wire I5. Such a separator would prevent parts of the fuse link, after the same has become blown or ruptured, from coming in contact with the lead wire I or cause the gases emitting from this fuse link to settle upon the glass globe of the lamp as may possibly reduce its light emitting qualities. By providing this paper partition 22 with an outer reiiecting surface, it will somewhat ald in reecting the light rays from the lamp through the mica window I I.

In assembling this plug fuse, the rivet 8 is mst inserted within the opening 3 of the casing I `and the inner end thereof is bent over to se- -end of the fuse link between the outer surface of the casing I and the shell 9 to form a good electrical contact, as is illustrated in Fig. 3.

I then prepare a powdered mixture containing forty parts of Pliolite, forty parts of sand and twenty parts of graphite and this mixture is poured within the opening 20, completely nlling the same and the recess 2l. To this mixture, I add a few drops of toluol or benzene until a plastic mass is formed. The lead wire it is then inserted into or embedded within this plastic mass. as is illustrated in Fig. 2 of the drawings. The paper partition 22 is then inserted within the exposive chamber 2, in the manner as illustrated, and thereafter, the opposite ends of the lamp i3 are caused to be seated within the recesses il and ill. Finally, the mica disc il is placed over the lamp and retained in position by crimping the metal cap i2 about the annular flange E.

The toluol or benzene is allowed to evaporate before closing the fuse casing by applying the mica disc thereto, whereat, a hard mass results having firmly attached thereto the wire i4 and the shell 9, establishing between the latter a suitable leak resistance. This leak resistance will have an electrical resistance rating of fifty thousand (50,000) ohms or more or sufcient resistance so that when the fuse plug is applied to an ordinary lighting circuit of 110 volts and the fuse link I0 becomes blown or ruptured, caused by a short circuit within the lighting circuit, suflicient current cannot pass as could possibly cause a fire or damage in the lighting circuit or at the point where the short circuit occurs, but will pass an innitesimal amount or sumcient current to keep the lamp I3 operative or glowing. It is, of course, to be understood that immediately after the fuse link I0 has blown, suiiicient current or a potential of forty volts or more will first pass to the lamp to cause the same to strike or glow and, thereafter, only suiiicient leakage is permitted as to cause the lamp to stay lighted after it has once started to glow.

To increase the electrical resistance rating, the amount of graphite may be reduced and the amount of graphite removed can be replaced with sand. My method of applying the resistor to the fuse will obviously cause all succeeding resistors to greatly vary in their electrical resistance ratings, but these resistors need not all be of the same constant ratings, as the lamp I3 will properly function as an indicator for fuses when used in connection with electrical circuits of 110 volts of any resistance rating between 50,- 000 and 1,000,000 ohms.

Instead of adding a solvent such as benzene or toiuol to the powdered mixture contained in the opening 20 of the fuse casing, the latter may be caused to pass through an oven of at least 300 F. where the Pliolite will become plastic and after leaving the oven, this plastic mass will be cooled, which results in a. solid mass having the metallic conductive terminals securely attached thereto without the danger of corrosion because of chemical changes and in electrical contact with the conductive material or graphite contained within the same.

The above powdered mixture containing Pliolite, graphite and sand may be flrst heated to form a plastic mass and then allowed to cool. This resultant solid mass may then be ground or milled into powdered form and finally poured lwithin the opening 20 of the fuse casing, where it can be again reheated or a solvent added thereto for rendering the same plastic for carrying out either of the steps as above outlined.

Still another way oi applying the leak resistance to the fuse is to iirst heat a mixture containing Pliolite, graphite and sand to form a plastic mass. Thereafter, this plastic mass is allowed to cool and then ground or milled into powdered form. This powdered mixture is then relieated to again form a plastic mass and while in this latter state is inserted within the opening preferably by a pressure gun until the opening 20 and recess it are completely lled. After this inserted plastic mass is allowed to cool, a hard, homogeneous product is obtained having the wire i6. embedded therein and the shell 9 attached thereto.

Instead oi employing the separator 22 within the explosive chamber of the fuse casing, I can and do apply a plastic cement `23 around the inner end of the rivet il arranged within the explosive chamber 2, over the lead wire i5 as at 22 so as to retain the latter within the groove I0, within the upper end 'of the vent opening 4 as at 25 so as to close the latter, over the top of the resistance compound seated within the opening 20 as at 26 and over the opposite ends of the lamp as at 2l in order to lock the latter within the recesses l'I and I8. By so applying this plastic cement, the explosive chamber is so closed that no lint or clust may enter the same but should the fuse link become blown or ruptured, the gases can escape from the explosive chamber l by passing around the upper edges of the casing below the cap l2 where it may escape intothe atmosphere. The pressure of the gases generated by the blowing of the fuse link is suiicient to separate the mica disc II and the cap I2 from the upper end of the fuse casing in order that the gases may escape therebetween but it is not sufficient in any manner to injure the successful operation of the glow lamp. The moten metal formed by the rupture of the fuse .link may settle upon the lower face of the glass tube of the lamp but this latter will not in any manner ailect the light rays from the lamp passing through the mica disc II.

In the form of invention shown in Fig. ll of the drawings, I place the plastic resistor coinposition within the bottom of the explosive chamber 2 as at 28 so that it may serve not only as a leak resistance, but will form a seal about the rivet 8 and establish means for connecting the lower end of the lead wire I5 and the fuse link I0 to this central contact terminal or rivet 8. The lead wire I4 of the lamp extends through the opening 20 and recess 2l where it is connected to the threaded contact shell 9. Plastic cement 29 lls the recess 2I for sealing the opening 20 and retaining the outer extremities of the wire I4 in a position whereat it may have a wedglng contact with the threaded shell 9.

The lower face 30 of the mica disc II is covered with a translucent paint the color of which will be visible from the exterior of the fuse and when the lamp I3 glows, rays of light will emit from the mica window Il of the color contained in the paint applied thereto. Instead of giving the mica window the effect of colored glass by applying paint thereto. I can insert a sheet of gelatine of the desired color either between the outer face of the mica disc and the metal cap I2 or between the'lower face of the mica disc and casing l. Also, it is possible to use colored transparent or translucent discs, sheets or other forms of covers made from glass or polymerized methyl methacrylate and similar molding composition.

The window of a fuse plug having a 5 amperage rating could be of a yellow color; for 1Q amperage fuses, the color could be red; for a 15 amperage fuse, the color could be blue; for a 20 amperage fuse, the color could be purple; for a 25 amperage fuse, the color could be brown, and the color of the window for a 30 amperage fuse could be green. Upon inspection, one would see a colored spot which would instantly announce the exact amperage rating of the fuse and when the fuse link glows, the lamp will project a colored signal for not only indicating the blown or inoperative condition of the fuse plug but for announcing the amperage rating of the fuse, so that a fuse of the correct or similar amperage rating may be ordered and used for replacing the blown fuse.

From the foregoing, it will be apparent that I can, by a slight variation in the construction and design of a conventional popular priced fuse plug, add a lamp indicator thereto and with the use o1' the improved resistor composition, make it possible to retail to the public an indicating plug fuse at a price approximating that of the conventional popular `priced fuses.

Manifestly, the construction herein shown is capable of considerable modification and such modifications as come within the scope of my claims, I consider within the spirit of my invention.

I claim:

l. An indicating plug fuse comprising a cupshaped casing molded from insulating material and having an explosive chamber formed therein with an upper opened end, said casing having a vent opening formed therein, a central contact terminal, a threaded contact terminal, a. fuse link arranged within the explosive chamber of said casing, extending through said vent opening and having its opposite extremities electrically connected to said contact terminals, a transparent disc closing the upper opened end of said explosive chamber, said chamber having recesses formed therein arranged diametrically opposite one another and at right angles to said vent opening, a glow lamp having its opposite ends seated within said recesses formed within said casing and arranged below said transparent disc, lead wires extending from said lamp, one of said lead wires being electrically connected to said central contact, said casing having an opening formed therein providing means for communication between one of the recesses formed within said casing and said threaded contact terminal, and a leak resistance arranged within and closing said opening for electrically connecting the other lead wire of said lamp with said threaded contact terminal.

2. An indicating plug fuse comprising a cupshaped casing molded from insulating material and having an explosive chamber formed therein with an upper opened end, said casing having a vent opening formed therein, a central contact terminal, a threaded contact terminal, a fuse link arranged within the explosive chamber of said casing, extending through said vent opening and having its opposite extremities electrically connected to said contact terminals, a transparent disc closing the upper opened end of said explosive chamber, said chamber having recesses formed therein, arranged diametrically opposite one another and at right angles to said vent opening, a glow lamp having its opposite endsseated within said recesses formed within said casing and arranged below said transparent disc, lead wires extending from said lamp, one of said lead wires being electrically connected to said central contact, said casing having an opening formed therein providing means for communication between one of the recesses formed within said casing and said threaded contact terminal, a leak resistance arranged within and closing said opening for electrically connecting the other lead Wire of said lamp with said threaded contact terminal, and a partition arranged within the explosive chamber of said casing for separating said fuse link from said lamp and lead wires thereof.

3. An indicating plug fuse comprising a cupshaped casing molded from insulating material and having an explosive chamber formed therein with an upper opened end, said casing having a vent opening formed therein, a central contact terminal, a threaded contact terminal, a fuse link arranged within the explosive chamber of said casing, extending through said vent opening and having its op-posite extremities electrically connected to said contact terminals, a transparent disc closing the upper opened end of said explosive chamber, said chamber having recesses formed therein, arranged diametrically opposite one another and at right angles to said vent opening, a glow lamp having its opposite ends seated within said recesses formed within said casing and arranged below said transparent disc, lead Wires extending from said lamp, one of said lead wires being electrically connecting to said central contact, said casing having an opening formed therein providing means for communication between one of the recesses formed within said casing and said threaded contact terminal, a leak resistance arranged within and closing said opening for electrically connecting the other lead wire of said lamp with said threaded contact terminal, a paper disc seated Within said explosive chamber and extending about said fuselink for separating the latter from said lamp and said paper disc having a reflecting surface arranged adjacent said lamp whereby the rays of light from said lamp may be reflected through said transparent disc.

4. An indicating plug fuse comprising a cupshaped casing molded from insulating material and having an explosive chamber formed therein with an upper opened end, said casing having a vent opening formed therein, a central contact terminal seated Within said casing, a threaded contact terminal, a fuse link arranged within the explosive chamber of the casing, extending through said vent opening and having its opposite extremities electrically connected to said contact terminals, a transparent disc closing the upper end of said explosive chamber, said casing having recesses formed therein and arranged diametrically opposite one another and at right angles to said vent opening, a glow lamp having its opposite ends seated within said recesses formed within said casing and arranged below said transparent disc, lead wires extending from said lamp, one of said lead wires being electrically connected to said contact terminal, said casing having an opening formed therein providing means of communication between one of said recesses formed therein and said threaded contact terminal, a leak resistance arranged within said casing and closing said opening for electrically connecting the other lead wire of said lamp with said contact terminal, a groove formed within said casing for receiving said lead Wire of the lamp connected to said centra] contact terminal and plastic cement covering said lead wire of the lamp arranged Within said groove, the upper end of said leal; resistance, said vent opening and central contact terminal whereby said explosive chamber Will Toe completely closed except for its upper opened end.

5. .An indicating plug inse comprising a casing having an explosive chamber arranged therein and an outer opened end communicating-with said explosive chamber, contact terminals, a fuse link arranged within said explosive chamber and connected iny series with said contact terminals, said casing having a pair of recesses arranged diametrically opposite and formed therein adjecent the opened end thereof, a glow lamp, the opposite ends of said glow lamp being seated within said recesses for supporting the same within the casing, means for connecting saidl glow lamp in series with said contact terminals but in parallel with. said fuse link and a mica window for closing the outer opened end oi said casing.

im; I f 

